In a photolithography process, in order to prevent dust or the like from being attached to a mask or a reticle, a pellicle having a pellicle film extended along one end of a frame of a size appropriate to enclose a mask pattern is used. In a state where such a pellicle is attached to the mask, the inside of the pellicle is highly airtight, and thus the pellicle film, which is thin, may sag or swell by a change in the air pressure or the temperature. When the pellicle film loses the flatness thereof in this manner, the optical characteristics of the pellicle film are changed. In the case where the pellicle film is protruded or recessed significantly, the pellicle film may contact the mask or hit the lid of a case accommodating the pellicle and thus is damaged.
As a solution to such a problem, Japanese Utility Model Publication for Opposition No. S63-39703, for example, describes a pellicle as described below. At least one vent hole is formed in a pellicle frame, and at the vent hole, a filter member inhibiting passage of dust or the like is provided in a space enclosed by a frame member, a mask substrate and a pellicle film so as not to come off. Japanese Laid-Open Patent Publication No. H05-113658 describes a pellicle in which a pellicle frame has an opening in at least a part of a side surface thereof and an inner surface thereof is pressure-sensitive. Japanese Laid-Open Patent Publication No. H05-107747 describes a pellicle including a cavity in a frame so as to capture foreign objects contained in gas passing the cavity.
Meanwhile, in a pellicle including a filter, the filter needs to have fine texture in order to capture microscopic dust or the like with certainty. This causes a problem that gas passes the filter at a low speed and thus it requires a long time to alleviate the pressure difference. In order to solve this, Japanese Laid-Open Patent Publication No. 2003-107678 describes a pellicle as described below. A pellicle frame is divided into an inner member and an outer member. The inner member is formed of natural color anodized aluminum or a ceramic material, which is highly lightfast, and the outer member is formed of aluminum alloy 7075-T6, which has a high proof stress. Two vent holes each having three bending portions are provided inside the pellicle frame, and a pressure sensitive layer for adsorbing dust is provided on an inner surface of the vent hole.
Currently, the wavelength used for lithography is progressively shortened, and EUV lithography has been developed as a next-generation lithography technology. EUV light refers to light of a wavelength in a soft X-ray region or a vacuum ultraviolet region, and is a light beam of about 13.5 nm±0.3 nm. In photolithography, the limit of pattern resolution is about ½ of the exposure wavelength. Even if an immersion method is used, the limit of pattern resolution is considered to be about ¼ of the exposure wavelength. Even if an immersion method with ArF laser (wavelength: 193 nm) is used, the exposure wavelength is expected to be about 45 nm at the minimum. Therefore, EUV lithography is expected as a revolutionary technology that realizes a significant size reduction as compared with the conventional lithography.
In the meantime, EUV light is easily absorbed to any substance. Therefore, in EUV lithography, the inside of an exposure device needs to be in vacuum. EUV light has a refractive index of close to 1 with respect to any substance. Therefore, a refraction optical system using a lens and a transmissive photomask is not usable unlike in the conventional lithography using visible light or ultraviolet light. For this reason, EUV lithography requires exposure to be performed in a reflection optical system using a reflective photomask and a mirror. As described in, for example, Japanese Laid-Open Patent Publication No. 2014-160752, a reflective photomask includes a multi-layer reflective film including a Mo layer and an Si layer stacked alternately a plurality of times on a substrate, and also includes an absorption layer, absorbing EUV light, provided on the reflective film.